1. Field of the Invention
This invention relates to a piezoelectric ceramic composition that is widely utilized in the field of actuators, sensors, resonators, and the like.
2. Description of the Related Art
The piezoelectric materials (piezoelectric ceramic compositions) possess an effect of generating strain when an electric field is applied thereto from outside (conversion of electric energy to mechanical energy) and an effect of generating an electric charge on the surface thereof when stress is applied thereto from outside (conversion of mechanical energy to electric energy) and have been widely utilized in various fields in recent years. Such a piezoelectric material as lead titanate zirconate (Pb(Zr, Ti)O3:PZT) excels in minute positional justification and finds utilization as in optical minute justification because it generates strain substantially proportional to the order of 1×10−10 m/V in response to an applied voltage. In contrast thereto, the piezoelectric material is utilized also as a sensor for reading out minute force and deformation because it generates a large electric charge in proportion to the stress exerted thereon or the amount of deformation of itself caused by the stress. Further, since the piezoelectric material possesses excellent responsiveness, it is capable of enabling the piezoelectric material itself or an elastic body adapted for union with the piezoelectric material to be excited and consequently allowed to induce resonation therewith and, therefore, is utilized as a piezoelectric transformer, a supersonic motor, etc.
Most piezoelectric materials now available for practical applications are solid solution systems (PZT systems) consisting of PbZrO3 (PZ)—PbTiO3 (PT). The reason for this fact is that an excellent piezoelectric property can be obtained by using a composition nearing the morphotropic phase boundary (M.P.B.) of the rhombohedral crystal-based PZ and the tetragonal crystal-based PT. The PZT-based piezoelectric materials that are adapted to suit various uses in consequence of the addition of a varying auxiliary component or additive have been developed widely. They are varied to such an extent of embracing use as an actuator for positional justification requiring a large voltage coefficient (d) instead of manifesting a small mechanical factor of merit (Qm) and expected to produce a large displacement in the application using direct current and use as a supersonic wave generating device like a supersonic motor possessing a large mechanical factor of merit (Qm) instead of manifesting a small voltage coefficient (d) and suiting the way of using an alternating current.
Also other piezoelectric materials than the PZT-based materials have been developed for practical applications. They are mostly solid solutions that have as main components such lead-based perovskite compositions as lead magnate niobate [Pb(Mg, Nb)O3:PMN].
The piezoelectric materials developed for practical applications are invariably lead-based piezoelectric materials as described above and have lead oxide (PbO) extremely rich in volatility even at low temperatures contained as a main component in a large amount nearing 60 to 70 mass %. PZT or PMN, for example, contains lead oxide in an amount of about ⅔ in mass ratio. The lead-based piezoelectric materials that contain lead in such a large amount entail many problems such as environmental resistance like public nuisance and ecological unease. During the manufacture of a lead-based piezoelectric material, for example, an extremely large amount on the industrial level of lead oxide is suffered to volatilize and diffuse into the air in the step of a thermal treatment such as firing when the products are ceramic articles or melting when the products are single crystal articles. Though the lead oxide emitted in the step of manufacture may be recovered, the lead oxide contained in piezoelectric products marketed as commercial articles is difficult of recovery in the present situation. When this lead oxide is widely released in the environment, the elution of lead caused by acid rain arouses anxiety. In consideration of the spread of the fields of application of piezoelectric ceramic materials and single crystals and the growth of the amounts of their application, therefore, the liberation of lead from the piezoelectric materials proves to constitute an extremely important problem.
As piezoelectric materials that contain absolutely no lead, barium titanate (BaTiO3) and bismuth-bedded ferroelectric materials have been known. The barium titanate, however, is devoid of serviceability in view of applications that involve joining with solder and mounting on a vehicle because it has such a low Curie point as 120° C. and suffers loss of piezoelectricity at a temperature exceeding it. On the other hand, the bismuth-bedded ferroelectric materials, though usually possessing a Curie point exceeding 400° C. and excelling in thermal stability, entail the problem in terms of productivity because it possesses large crystal anisotropy and requires spontaneous polarization to be oriented as by hot forging. Generally, the elimination of lead in a piezoelectric material leads to degradation of piezoelectric property. When the lead content is thoroughly eliminated from the conventional piezoelectric material, for example, it is judged that large piezoelectricity is obtained with difficulty.
Further, in search of a new piezoelectric material, studies have been being promoted on the sodium bismuth titanate-based materials in recent years. JP-B-4-60073 and JA Hei 11-180769, for example, disclose materials that contain sodium bismuth titanate and barium titanate and JP-A-11-171643 discloses materials that contain sodium bismuth titanate and potassium bismuth titanate. Then, JP-A-16-035350 discloses systems that contain sodium bismuth titanate and sodium bismuth, potassium bismuth titanate, and a third component.
These sodium bismuth titanate-based materials, however, fail to obtain sufficient piezoelectric property as compared with lead-based piezoelectric materials and consequently require further improvement in the piezoelectric property in the factual state of affairs. In this situation, the present patent applicant has proposed a piezoelectric ceramic material that contains a first compound possessing a rhombohedral crystal-based perovskite structure, a second compound possessing a tetragonal crystal-based perovskite structure, and a third compound including bismuth (Bi), a divalent metallic element such as magnesium (Mg), a tetravalent metallic element such as zirconium (Zr), and oxygen (O) (refer to JP-A-2005-47745, JP-A-2005-47746, and JP-A-2005-47748). He has also proposed a piezoelectric ceramic article that contains a first compound possessing a rhombohedral crystal-based perovskite structure, a second compound possessing a tetragonal crystal-based perovskite structure, and a third compound including bismuth, iron (Fe), a pentavalent metallic element like tantalum (Ta), and oxygen (O) (refer to JP-A-2005-47747). The piezoelectric ceramic articles disclosed in JP-A-2005-47745, JP-A-2005-47746, and JP-A-2005-47748 are capable of amply improving such piezoelectric properties as displacement and copiously enhancing the applicability of a non-lead-based piezoelectric material.
Incidentally, as piezoelectric materials, while materials abounding in piezoelectric constant (d) and manifesting large displacement are being sought, materials that possess a large Qm (mechanical factor of merit: index of energy loss) in applications involving an alternating current are being sought. In the resonator driven with high frequency, for example, the fact that a given piezoelectric material has small Qm constitutes a factor for generation of heat, for example. In the case of a piezoelectric device required to generate vibration of a large amplitude, the fact that the Qm of a given piezoelectric material is small possibly leads to destruction of the device.
As considered from the viewpoint of the Qm value mentioned above, the inventions disclosed in the foregoing patent documents have given virtually no study to the Qm value in the factual state of affairs. The inventions disclosed in the preceding patent documents invariably have placed emphasis on such displacement properties as piezoelectric constant (d). The piezoelectric ceramic articles disclosed in Patent Documents 5˜8, for example, have given no study to the Qm.